The invention relates to pressure exchangers where a liquid under a high pressure hydraulically communicates, through a working liquid, with a lower pressure, second liquid, and transfers pressure between the liquids. More particularly, the invention relates to cavitation control and anti-cavitation elements, especially in rotary pressure exchangers.
Many industrial processes, especially chemical processes, operate at elevated pressures. These processes require a high pressure feed, and produce a high pressure product (including high pressure effluents). One way of obtaining a high pressure feed to an industrial process is by feeding relatively low pressure feed through a pressure exchanger to exchange pressure between the high pressure effluent and the low pressure feed. One type of pressure exchanger is a rotary pressure exchanger. Rotary pressure exchangers have a rapidly rotating rotor with channels through the rotor to allow hydraulic communication between the high pressure liquids and thereafter the low pressure liquids, through the working liquid.
U.S. Pat. No. 4,887,942, U.S. Pat. No. 5,338,158, and U.S. Pat. No. 5,988,993, all three of which are incorporated herein by reference, discuss rotary pressure exchangers of the general type described herein, for transferring pressure energy from one fluid to another. This type of pressure exchanger is a direct application of Pascal""s Law, which may be stated as xe2x80x9cPressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel.xe2x80x9d Pascal""s Law means that if a high pressure fluid is brought into hydraulic contact with a low pressure fluid, the pressure of the high pressure fluid is reduced, the pressure of the low pressure fluid is increased, and the pressure exchange is accomplished with minimum mixing. The pressure exchanger applies Pascal""s Law by alternately and sequentially
(1) bringing a channel, which contains a low pressure working liquid, into hydraulic contact with a first chamber containing high pressure liquid, thereby depressurizing the liquid in the chamber, and pressurizing the working liquid in the channel; and
(2) bringing the channel, which now contains high pressure working liquid, into hydraulic contact with a second chamber containing low pressure liquid, thereby pressurizing the low pressure liquid in the second chamber and depressurizing the high pressure working liquid in the channel.
The net result of the pressure exchange process, in accordance with Pascal""s Law, is to cause the pressures of the two fluids to approach one another. The result is that, in a chemical process operating at high pressures, e.g., 950-1000 psi, where the feed is generally available at low pressures, e.g., atmospheric pressure to about 50 psi, and the product is available from the process at 950-1000 psi, the low pressure feed and the high pressure product are both fed to the pressure exchanger to pressurize fresh feed and depressurize product. The industrially applicable effect of the pressure exchanger on an industrial process is the reduction of high pressure pumping capacity needed to raise the feed to high pressures. This can result in an energy reduction of up to 65% for the process and a corresponding reduction in pump size.
In a rotary pressure exchanger, a rotor carries the working liquid in a channel, and the rotation of the rotor provides alternating hydraulic communication of the working liquid in the channel with the high pressure liquid in the chambers exclusively, and, a short interval later, with the low pressure liquid in the chambers exclusively. The channel has openings at each end, one opening for hydraulic communication with the first chamber, and one opening for hydraulic communication with the second chamber. Because of the countercurrent flow of the two feed streams, the initially high pressure feed and the initially low pressure feed streams, in the manifolds, the channel is in hydraulic communication with high pressure liquid and thereafter with low pressure liquid.
Rotary pressure exchangers have a rapidly rotating rotor with a plurality of substantially longitudinal channels extending through the rotor. These channels allow many very brief intervals of hydraulic communication through the working liquid in the channel between the two liquids. The two liquids are otherwise hydraulically isolated from each other. There is minimal mixing or leakage in the channels. This is because the channels have a zone of relatively dead liquid, the working liquid, as an interface in the channels between the two liquids. This permits the high pressure liquid to transfer its pressure to the lower pressure liquid, thereby exchanging pressure between the liquids.
The rotor is present in a cylindrical housing, with the end elements of the exchanger having end plates with openings for mating with the channels in the rotor so as to be alternately in hydraulic communication with high pressure working liquid in one channel and subsequently low pressure working liquid in another channel, and being sealed off from the channels between the intervals of hydraulic communication, as the channels rotate.
The rotor in the pressure exchanger is supported by a hydrostatic bearing and driven by either the flow of fluids through the rotor channels and exchanger manifolds or a pump motor. In order to accomplish this, extremely low friction is required. For this reason the pressure exchanger does not use rotating seals. Instead, fluid seals and fluid bearings are used. Extremely close tolerance fits are used to minimize leakage. In use, internal leakage constantly occurs from higher-pressure areas to lower pressure areas, but, absent cavitation, the amount of internal leakage is generally constant over the operating range of the pressure exchanger, and this internal leakage has minimal to no effect on the downstream industrial process, other than to marginally lower the overall efficiency of the downstream process.
In most applications of pressure exchangers, the pressure exchangers are used with low viscosity, incompressible fluids, e.g. water. Any abnormal internal leakage between areas with high and low pressure, especially leakage associated with cavitation, cavitation damage, and cavitation erosion, substantially reduces hydraulic efficiency in the exchanger. If this leakage becomes uncontrolled, for example, as the result of vibrations and acoustic waves from cavitation, it can lead to still more cavitation at the outlet, especially if the sealing surfaces are not functioning satisfactorily, with a severely reduced working life as a consequence. Furthermore, any dramatic change in pressure, such as the fluid sees as it moves from high to low pressure areas in the end plates, can create cavitation.
Because of the high pressure drops involved, the high rotational speeds involved, and the closeness of the elements, typically on the order of microns to tens of microns, the rotary pressure exchanger is highly susceptible to cavitation and to damage from cavitation, such as, cavitation erosion, and power robbing vibrations. The high pressure drops, close tolerances, and high rotational velocities all contribute to the need for effective cavitation control.
xe2x80x9cCavitationxe2x80x9d as used herein is the formation and collapse of vapor cavities in a flowing liquid. Cavitation occurs whenever the local pressure is quickly reduced to or below that of the liquid""s vapor pressure. The formation and instantaneous collapse of innumerable tiny cavities or bubbles within a liquid characterize cavitation, especially when the liquid is subjected to rapid and intense changes in pressure. One adverse effect of cavitation is xe2x80x9ccavitation erosion.xe2x80x9d In cavitation erosion, the cavities pit and erode the surface where they form. Another adverse effect of cavitation is the noise and vibration associated with bubbles forming and bursting, especially when such noise and vibration occurs in narrow fluid seals.
The cavitation potential of end clearance leakage outflow of the low pressure side is a limiting design factor. It is therefore highly desirable to reduce the cavitation susceptibility of the outlets of the rotor channels and end plate apertures. And, it is to these ends that the present invention is directed.
According to the invention, cavitation is controlled and substantially eliminated by the controlled bleeding and shunting of high pressure liquid in a channel to either an appropriate liquid seal or a lower pressure channel. The structure and apparatus of this invention substantially reduces cavitation, and associated problems, such as cavitation erosion, pitting, vibration, and noise in devices such as pressure exchangers which transfer pressure from a high pressure liquid to a low pressure liquid, and therefore, it reduces the need for increased pumping power. The pressure exchanger transfers pressure between a high pressure liquid feed and a low pressure liquid feed in a pressure exchanger system that includes a housing with two end covers. Each end plate has an inlet and an outlet aperture. The apertures of one end plate are aligned with the apertures of the opposite end plate to allow pressure exchange between the liquids in the manifolds. A cylindrical rotor is inside the housing and is arranged for rotation about the housing""s longitudinal axis. The rotor has a number of through-going channels with openings at each end arranged symmetrically about the longitudinal axis. While the channels are arranged symmetrically about the longitudinal axis of the rotor, they may be offset from parallel longitudinal alignment with the longitudinal axis of the rotor to capture angular momentum and provide angular velocity to the rotor. The rotor""s channels are arranged for periodic hydraulic communication with a pair of apertures, one in each end plate, in such a manner that during rotation they alternately expose fluids at high pressure to each other and thereafter fluids at low pressure to each other through the working fluid in the channel. The end plates"" or end covers"" inlet and outlet apertures are designed with perpendicular flow cross sections in the form of segments of a circle. An anti-cavitation structure, in the form of a recess, groove, or recessed channel is present in either one or both of the end plates.
In the rotary pressure exchanger of the invention, the structure for controlling and eliminating cavitation is part of the end plates and provides a pressure change in the channel while the channel is blocked by the end plates. This partially depressurizes the channel. The structure may be in the form of one or more grooves, where the grooves are positioned to provide hydraulic communication between the openings of the channels and the liquid seal between the rotor and the end piece. There may be one or more grooves in the end plates joining openings of the channels with the liquid seal between the rotor and the end piece to relieve pressure and prevent cavitation. The grooves are recessed into the end plate.
According to the invention one or more grooves recessed into the end plates hydraulically connect to the channels and allow for a bleed of pressure from the channels. For example, in one aspect the end plate has one or more anti-cavitation recessed grooves periodically connecting to channel outlets in the rotor and bleeding fluid and pressure to the liquid seal volume between the end cap and the rotor. In another aspect of the invention, the end plate has one or more anti-cavitation recessed grooves hydraulically joining the inlets/outlets of appropriate channels in the rotor to bleed or shunt high pressure and high pressure fluid both to a low pressure rotary channel and to the liquid seal volume between the end piece and the rotor.